Unlike liquid fuel, which consistently holds about the same volume of fuel across a broad range of conditions, the amount of gaseous fuel inside a storage vessel depends on the pressure and temperature of the gas. The problem of “under-filling” is a phenomenon whereby a vessel is filled with less-than-desired mass of gaseous fuel. While filling the vessel rated for a certain pressure, the gas temperature rises during the filling process [1]. This temperature rise reduces the density of the gas and packs fewer gas molecules in the vessel than what would be possible had the temperature remained constant. As the temperature of the gas in the cylinder equilibrates with the environment after the filling process is complete, the user has less mass of gas in the vessel than what the rated pressure and initial temperature would have allowed. This essentially results in an under filled vessel relative to its rated specification.
Under-filling of gaseous fuel during a filling process causes the vehicle user to experience a reduced driving range. Typically the temperature rise is compensated for in the fuelling station dispenser by transiently over-pressurizing the tank. The amount of over-pressurization is dictated by the capabilities of the filling station, the pressure differential between the dispenser of the filling station and the vessel's pressure, ambient temperature and the design capacity allowed for in the vessel to handle pressure higher than its rated designation. Currently there is no means to compensate for the under-filling problem by thermal management of the heat generated during the filling process.
Despite current understanding of the problems associated with the temperature increase during a gas-fill process, to date no adequate method or device is available that can reduce the amount of temperature increase during a gas-fill process, in particular during a fast-fill process.